Summary Neuroplasticity is key to rehabilitation from visual deficits. Yet, visual neural plasticity in adults remains poorly understood. This is especially true in primary visual cortex (V1), where methods for producing long-lasting adult neural plasticity have yet to be thoroughly established. The proposed research aims to overcome this roadblock by investigating the neural bases of the McCollough Effect (ME), a visual illusion produced by viewing colored, oriented patterns that causes black and white patterns to appear colorful. The ME can endure for weeks, indicating that it produces long-lasting changes in the visual system. The neural loci and mechanisms producing it remain under debate, however. Behavioral work suggests that the ME likely arises at a very early level of visual processing. However, past neuroimaging results have failed to support this claim, and the specific neural subpopulations that produce ME have yet to be identified. The current proposal contains a comprehensive investigation of the ME using fMRI to clarify its poorly understood neural bases. In Aim 1, multivariate pattern analysis will test whether neural populations underlying the ME are located in V1. A classifier will be trained to distinguish patterns of activity in V1 arising from black and white vs colorful patterns, using data gathered prior to induction of the ME. If V1 is the locus of the ME, then following its induction, the same classifier should categorize responses to black and white patterns as colorful, matching perception. Aim 2 will use ultra-high-resolution fMRI to test whether effects of an ME induced in one eye localize to ocular dominance columns containing neurons selective for that eye, as suggested by behavioral work. The high-resolution data will be further characterized using a forward modeling technique that disentangles the relative responses and gain changes of different color-sensitive neural populations. Together, these experiments will test the hypothesis that the McCollough Effect arises from gain changes in populations of eye- and color- selective neurons in V1, demonstrating feed-forward neural plasticity in adult primary visual cortex. This work is part of a research training and career development plan for Dr. Katherine E.M. Tregillus, conducted at the University of Minnesota under the co-sponsorship of Dr. Stephen Engel and Dr. Cheryl Olman. The work is a logical extension of previous research from the three investigators, and the University of Minnesota provides an ideal environment for carrying it out. The proposed research will advance understanding of neuroplasticity and provide the basis for the applicant?s independent career, where she intends to translate her work on long-term adaptation to clinical applications. For example, paradigms similar to the ME could be developed to balance the gain of the two eyes in amblyopia, or to improve the gain of color-opponent signals in color anomalous observers.